Experiments with Supersonic Beams as a Source of Cold Atoms

viability for molecules essential.
In the experiment described here, oxygen is slowed from an initial velocity
of 389 m/s to a 83 m/s, removing more than 95% of the initial kinetic energy. The
apparatus used for this is almost identical to the apparatus used to stop metastable
neon, and the coilgun used is exactly the same. As such, only the differences between
the experiments (in beam creation and detection) are discussed.
4.6.1 Oxygen’s Magnetic Levels
At this point, it is useful to consider how the rotational spin states of oxygen
affect its interaction with a magnetic field. The state that is slowed is the 3 Σ − g ground
state. Nuclear statistics forbid the existence of K =0, 2, 4,...rotational levels of 16 O2.
The two unpaired electrons of the oxygen molecule lead to an electron spin angular
momentum S = 1. The sum of spin angular momentum, S, and the rotational angular
momentum, K, gives three possible total angular momentum states J =0, 1, 2for
the K = 1 ground state (J = K + S, electron orbital angular momentum in the Σ
state is zero). In the weak-field Zeeman regime, the three total angular momentum
states each split into 3 sub-levels with different total angular momentum projections
in the direction of the magnetic field. At high fields (above 2.5 T) the electron spin
decouples from the rotational angular momentum and forms three different sets of
three levels, where each set is almost-degenerate. The J =2,MJ = 2 sub-level has
the highest magnetic moment both in the low and high field regions, which leads to
the magnetic moment being approximately the same in both the low and high field
regime. The magnetic moment of this sublevel is approximately equal to 1.8 Bohr
magnetons [86]. This magnetic sublevel has an avoided level crossing with the J =2,
MJ = 2 sub-level from the K = 3 manifold at about 8 T, changing its behavior from
low-field to high-field seeking, which is incompatible with the coilgun as it is currently
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